CoPtP thin film having very high perpendicular magnetic anisotropy and method for manufacturing the same

ABSTRACT

The present invention relates to a CoPtP thin film having very high perpendicular magnetic anisotropy and a method for manufacturing the same. The method for manufacturing a CoPtP alloy thin film according to the present invention includes the steps of preparing an electroplating solution including CoSO 4 ·7 H 2 O at 0.05˜0.2 M, H 2 PtCl 6  at 0.005˜0.02 M, and NaH 2 PO 2  at 0.01˜0.4 M, and dipping a basic material into the electroplating solution and forming the CoPtP alloy thin film on the basic material by electroplating at a low temperature.

This application claims priority to Korean Patent Application No. 10-2005-0069384, filed on Jul. 29, 2005, and all the benefits accruing therefrom under 35 U.S.C. § 119, and the contents of which in its entirety are herein incorporated by reference.

BACKGROUND OF THE INVENTION

(a) Field of the Invention

The present invention relates to a CoPtP thin film and a method for manufacturing the same, and more particularly to a CoPtP thin film having very high perpendicular magnetic anisotropy and a method for manufacturing the same.

(b) Description of the Related Art

A CoPtP alloy shows high coercivity and squareness in a perpendicular direction. Due to such properties, the CoPtP alloy is very suitable for an ultra high density perpendicular magnetic recording media which is used in the next generation hard disk drive. Ferromagnetic alloys such as the CoPtP alloy are largely demanded in a field of micro electro mechanical systems (MEMS). In particular, it is indispensable to manufacture ferromagnetic alloys such as the CoPtP when micro-scale elements or nano-scale elements such as a micro actuator or a magnetic valve etc. are realized. Therefore, various kinds of research regarding magnetic properties of the CoPtP alloy and controlling methods of a microstructure thereof have been developed.

The general CoPtP alloy has a face centered cubic (fcc) structure or an ordered face centered tetragonal (ordered fct) L1₀ structure. In particular, CoPtP alloy with the L1₀ structure has a property that magnetocrystalline anisotropy largely acts in a perpendicular direction. In this structure, Co atoms and Pt atoms are alternately packed in a (002) plane, which is perpendicular to a c-axis, of a tetragonal structure.

The ferromagnetic CoPtP alloy can be manufactured by a vacuum deposition method, an electroplating method, and so on. If the vacuum deposition method is used, the ferromagnetic CoPtP alloy is manufactured through phase transformation by heat treatment. Thereby, a phenomenon that a circuit is broken and so on by heat occurs. Therefore, the vacuum deposition method is not suitable for use for MEMS elements, micro elements, or nano elements.

On the contrary, the electroplating method has an advantage that a ferromagnetic CoPtP alloy can be directly manufactured without heat treatment. Since the electroplating method is carried out at a low temperature of 30˜55° C., it has been newly spotlighted as a method for realizing a magnetic recording media and MEMS elements.

If the CoPtP alloy thin film is manufactured by using the electroplating method, each component in the alloy is difficult to control due to an electrochemical potential difference between cobalt ions (Co²⁺) and platinum ions (Pt²⁺). To solve such a problem, the platinum ions have usually been produced by using a very complicated complex compound.

For example, a CoPtP alloy was manufactured with H₂PtCl₆, Na₂Co(P₂O₇)₂, Na₃PO₄, and NaH₂PO₂ reagents by the electroplating method by a research group including Professor K. Nobe of UCLA in the U.S.A. The amount of Pt was over 40.0 wt% and the coercivity was really 2960 Oe, and the squareness was in the range of about 0.3 to 0.4. These values are very low.

Meanwhile, a CoPtP alloy was manufactured with a Pt amino-nitrite complex and Co amino-citrate by a research group including Professor Cavallotti of Italy. The coercivity of the CoPtP alloy, which was in the range of about 3750 Oe to 4300 Oe, was larger than that developed by the research group including Professor K. Nobe.

However, there is a problem in that the above-mentioned methods are carried out at a temperature of over 60° C., and complicated complex compounds are used in manufacturing an electroplating solution. Therefore, it is difficult to obtain a thin film having stable quality and that can be actually adapted.

SUMMARY OF THE INVENTION

The present invention is contrived to solve the above problems, and to provide a method for manufacturing a CoPtP alloy thin film using a commercialized reagent without a complicated complex compound by an electroplating method.

In addition, the present invention is contrived to provide a CoPtP alloy thin film having good coercivity and squareness in a perpendicular direction by using a method for manufacturing the above-mentioned CoPtP alloy thin film.

The method for manufacturing a CoPtP alloy thin film according to the present invention includes steps of preparing an electroplating solution including CoSO₄·7H₂O at 0.05˜0.2 M, H₂PtCl₆ at 0.005˜0.02 M, and NaH₂PO₂ at 0.01˜0.4 M, dipping a basic material into the electroplating solution, and forming the CoPtP alloy thin film on the basic material by electroplating at a low temperature.

It is preferable that the electroplating solution further includes an ammonium salt at 0.1˜0.5 M, and that it further includes Na₄P₂O₇ at 0.3˜0.7 M. The basic material is preferably made of cobalt.

The electroplating is preferably carried out at a temperature of 30˜55° C. in the step of forming the CoPtP alloy thin film.

A CoPtP alloy thin film is manufactured by using the method for manufacturing a CoPtP alloy thin film according to the present invention.

The CoPtP alloy thin film preferably includes a plurality of CoPt grains and boundary layers surrounding the CoPt grains. The boundary layers preferably include Pt and P. Each of the CoPt grain is preferably separated from each other by the boundary layers.

The CoPtP alloy thin film preferably has coercivity of 4000 Oe ˜7000 Oe, and it preferably has squareness of not less than 0.7.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and advantages of the present invention will become more apparent by describing exemplary embodiments thereof in detail with reference to the attached drawings, in which:

FIG. 1 is a photograph of a CoPtP alloy thin film taken by a transmission electron microscope (TEM);

FIG. 2 is a magnetic hysteresis curve of the CoPtP alloy thin film which is manufactured according to Experimental Example 1 of the present invention;

FIG. 3 is a magnetic hysteresis curve of the CoPtP alloy thin film which is manufactured according to Experimental Example 2 of the present invention;

FIG. 4 is a magnetic hysteresis curve of the CoPtP alloy thin film which is manufactured according to Experimental Example 3 of the present invention; and

FIG. 5 is a magnetic hysteresis curve of the CoPtP alloy thin film which is manufactured according to Experimental Example 4 of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Now, exemplary embodiments of the present invention will be described with reference to the attached drawings in order for those skilled in the art to work out the present invention. However, the present invention can be embodied in various modifications and thus is not limited to the embodiments described below.

An electroplating solution is prepared in order to manufacture a CoPtP alloy thin film according to the present invention. The electroplating solution includes CoSO₄·7H₂O, H₂PtCl₆, and NaH₂PO₂.

CoSO₄·7H₂O at 0.05˜0.2 M is used for providing cobalt with the electroplating solution. If the amount of CoSO₄·7H₂O is less than 0.05 M, a concentration of cobalt, which is an important element for showing magnetic properties of an alloy, is decreased, and thereby coercivity of the alloy is reduced. If the amount of CoSO₄·7H₂O is more than 0.2 M, the concentration of cobalt is increased and thereby perpendicular magnetic anisotropy of the alloy is deteriorated. Therefore, the magnetic properties in a perpendicular direction are reduced.

In addition, H₂PtCl₆ at 0.005˜0.02 M is used in order to provide Pt with the electroplating solution. If the amount of H₂PtCl₆ is less than 0.005 M, the grain boundary is uniformly formed and thereby coercivity and squareness in a perpendicular direction are reduced. If the amount of H₂PtCl₆ is more than 0.02 M, cobalt existing in the grain is alloyed with Pt and thereby magnetic properties are is reduced.

Furthermore, NaH₂PO₂ at 0.01˜0.4 M is used in order to induce co-deposition of P. If the amount of NaH₂PO₂ is less than 0.01 M, the concentration of P cannot be sufficient for the precipitates of P in a grain boundary to act as a pinning mechanism. If the amount of NaH₂PO₂ is more than 0.4 M, it cannot influence the amount of P which is induced to be co-deposited to the alloy, but also precipitates are generated in a solution, and thereby it is difficult to manufacture a solution.

Since CoSO₄·7H₂O, H₂PtCl₆, and NaH₂PO₂ are not only inexpensive but are also easy to obtain, the electroplating solution can be easily manufactured.

In addition, other compounds can be further added to the electroplating solution if necessary. For example, an ammonium salt can be further added. NH₄Cl, NH₄OSO₂NH₂, NH₄OH, or NH₄NO₃ can be used as the ammonium salt. The ammonium salt is activated as ammonium ions (NH₄ ⁺) in the electroplating solution. The ammonium ions are combined with cobalt ions and platinum ions, and thereby forming complex ions. Therefore, the ammonium ions change the electric potential in which cobalt ions and platinum ions are plated in the electroplating solution. The combining amount and composition of the alloy are varied according to the amount of ammonium ions. For this, an ammonium salt at 0.1˜0.5 M is further added. If the amount of ammonium salt is less than 0.1 M, the amount of ammonium ions is small, and thereby it is not easy to form complex ions during electroplating. Therefore, electroplating of the alloy becomes more difficult. If the amount of ammonium salt is more than 0.5 M, the formation of the complex ions reaches a saturation level, so more ammonium ions are not necessary.

In addition, NaH₂PO₂ can be further added to the electroplating solution. NaH₂PO₂ is added to the electroplating solution in order to control pH of the electroplating solution and to act as a supporting electrolyte. For this, NaH₂PO₂ at 0.3˜0.7 M is added. If the amount of NaH₂PO₂ is less than 0.3 M, it is not sufficient to manufacture an alkali solution and pH supporting of the solution becomes unstable. If the amount of NaH₂PO₂ is more than 0.7 M, an unnecessary amount thereof is added to the electroplating solution and thereby the temperature should be raised during manufacturing of the electroplating solution and electroplating.

A basic material is dipped in the electroplating solution which is manufactured by the above-mentioned method. For example, the basic material can be cobalt, and other basic materials can also be used. The basic material is electroplated at a low temperature of 30˜55° C. If a water solution including CoSO₄·7H₂O, H₂PtCl₆, and NaH₂PO₂ is used, it is possible to electroplate at a low temperature. The electroplating is carried out as static current electroplating.

If the electroplating temperature is less than 30° C., the electroplating solution is precipitated. In addition, if the electroplating temperature is over 50° C., it is impossible to maintain an accurate concentration of the electroplating solution due to evaporation thereof. Since the basic material acts as an anode, a CoPtP alloy thin film is formed on the basic material.

By using the above-mentioned method, a CoPtP alloy thin film with coercivity of 4000 Oe˜7000 Oe and squareness of not less than 0.7 in a perpendicular direction can be manufactured. The CoPtP alloy thin film manufactured by using the above-mentioned method has good perpendicular magnetic anisotropy.

The squareness is a ratio of residual magnetization amount to maximum magnetization amount. The squareness is used as a parameter for representing a shape of a magnetic hysteresis curve. Generally, the energy product (BH) of the material becomes larger as the squareness thereof increases even if the material has the same coercivity and magnetic flux density. That is, a material with a high squareness can be a hard ferrite material having high capacity. Therefore, squareness is a very important parameter that determines whether the material can be used as a recording medium.

FIG. 1 shows a photograph of a CoPtP alloy thin film taken by a TEM. If the CoPtP alloy thin film is used in the magnetic recording medium, the grain size influences the size of a bit which is a unit of the magnetic recording medium. That is, some grains are gathered and are used as one bit. If the size of the grain becomes small, it is possible to further enhance the recording density. In particular, the recording density is further enhanced when a plurality of grains are well-separated from each other by boundary layers. The above-mentioned properties are shown in the photograph of a CoPtP alloy thin film of FIG. 1 as it is.

As shown in FIG. 1, the CoPtP alloy thin film includes a plurality of CoPt grains and boundary layers surrounding them. The CoPt grains are shown as a black color and the boundary layers are shown as a white color. The boundary layer includes Pt and P. The size of the CoPt grains is in the range of 7 nm to 20 nm. If the size of the CoPt grains is less than 7 nm, the coercivity thereof is reduced since thermal stability thereof is reduced. If the size of the CoPt grain is more than 20 nm, the coercivity is not only reduced but also the number of magnetic domains existing in the grain becomes large. As a result, magnetic properties are deteriorated.

As shown in FIG. 1, each of the CoPt grains is clearly separated by the boundary layers. Since the CoPtP alloy thin film has the above-mentioned micro structure, it has good perpendicular magnetic anisotropy. Therefore, the CoPtP alloy thin film is suitable for a perpendicular magnetization recording medium to be used in a hard disk drive.

The experimental examples of the present invention will be explained below. The experimental examples of the present invention are merely to illustrate the present invention, and the present invention is not limited thereto.

EXPERIMENTAL EXAMPLE 1

An electroplating solution including CoSO₄·7H₂O at 0.13 M, H₂PtCl₆ at 0.005 M, NaH₂PO₂ at 0.05 M, an ammonium salt at 0.15 M, and Na₄P₂O₇ at 0.45 M was prepared. The pH of the electroplating solution was 8.4 and the temperature thereof was maintained at 40° C. As an anode, a cobalt plate was dipped in the electroplating solution and a current density was maintained at 10 mA/cm². Therefore, a CoPtP alloy thin film was formed by using an electroplating method.

FIG. 2 shows a magnetic hysteresis curve of the CoPtP alloy thin film that was manufactured according to Experimental Example 1 of the present invention. It is possible to obtain values of saturated magnetization (Ms), residual magnetization (Mr), coercivity (Hc), squareness (Mr/Ms), etc., from FIG. 2. The X-axis denotes a magnetic field strength and the Y-axis denotes a magnetization amount according to the magnetic field strength in the magnetic hysteresis curve. In particular, the Y-axis is shown to be standardized by dividing a value of the magnetization amount by a value of saturated magnetization (Ms). The value of the coercivity is a point at which the magnetic hysteresis curve shown in FIG. 2 meets the X-axis, and the value of the squareness is a point at which the magnetic hysteresis curve shown in FIG. 2 meets the Y-axis.

FIG. 2 shows a magnetic hysteresis curve of the electroplated CoPtP alloy thin film which was measured in a perpendicular direction. The coercivity of the CoPtP alloy thin film was 4520 Oe and the squareness thereof was 0.83 in the magnetic hysteresis curve shown in FIG. 2. Therefore, good magnetic anisotropy was shown.

EXPERIMENTAL EXAMPLE

An electroplating solution including CoSO₄·7H₂O at 0.13 M, H₂PtCl₆ at 0.01 M, NaH₂PO₂ at 0.02 M, an ammonium salt at 0.25 M, and Na₄P₂O₇ at 0.5 M was prepared. The pH of the electroplating solution was 8.4 and the temperature thereof was maintained at 30° C. As an anode, a cobalt plate was dipped in the electroplating solution and a current density was maintained at 5 mA/cm². Therefore, a CoPtP alloy thin film was formed by using an electroplating solution.

FIG. 3 shows a magnetic hysteresis curve of the CoPtP alloy thin film which was manufactured according to Experimental Example 2 of the present invention. The coercivity of the CoPtP alloy thin film was 4410 Oe and the squareness thereof was 0.96. Since the squareness of the CoPtP alloy thin film is very high, it shows the best property among CoPtP alloy thin films which have been manufactured by using an electroplating method thus far.

EXPERIMENTAL EXAMPLE 3

An electroplating solution including CoSO₄·7H₂O at 0.10 M, H₂PtCl₆ at 0.015 M, NaH₂PO₂ at 0.02 M, an ammonium salt at 0.27 M, and Na₄P₂O₇ at 0.5 M was prepared. The pH of the electroplating solution was 8.4 and the temperature thereof was maintained at 40° C. As an anode, a cobalt plate was dipped in the electroplating solution and a current density was maintained at 5 mA/cm². Therefore, a CoPtP alloy thin film was formed by using an electroplating method.

FIG. 4 shows a magnetic hysteresis curve of the CoPtP alloy thin film which was manufactured according to Experimental Example 3 of the present invention. The coercivity of the CoPtP alloy thin film was 5550 Oe and the squareness thereof was 0.76. Considering the above squareness, the CoPtP alloy thin film shows the best property among CoPtP alloy thin films that have been manufactured by using an electroplating method thus far.

EXPERIMENTAL EXAMPLE 4

An electroplating solution including CoSO₄·7H₂O at 0.08 M, H₂PtCl₆ at 0.01 M, NaH₂PO₂ at 0.05 M, an ammonium salt at 0.2 M, and Na₄P₂O₇ at 0.45 M was prepared. The pH of the electroplating solution was 8.4 and the temperature thereof was maintained at 40° C. As an anode, a cobalt plate was dipped in the electroplating solution and a current density was maintained at 7 mA/cm². Therefore, a CoPtP alloy thin film was formed by using an electroplating method.

FIG. 5 shows a magnetic hysteresis curve of the CoPtP alloy thin film which was manufactured according to Experimental Example 4 of the present invention. The coercivity of the CoPtP alloy thin film was 6950 Oe and the squareness thereof was 0.79. The CoPtP alloy thin film has the highest value of coercivity among CoPtP alloy thin films that have been manufactured by electroplating thus far.

According to the present invention, a hard ferrite CoPtP alloy thin film can be manufactured at a low temperature by electroplating using commercialized reagents without complicated complex compounds. The perpendicular magnetic anisotropy of the CoPtP alloy thin film manufactured by using the above method is much better than that of the alloy of a prior art. In particular, it is suitable as a magnetic material for a perpendicular magnetization recording medium considering a micro structure and a magnetic property thereof.

The method for manufacturing a CoPtP alloy thin film according to the present invention does not need a following process after electroplating. In particular, a process thereof is simple and stability of performance is improved.

Although the exemplary embodiments of the present invention have been described, it can be easily understood by those skilled in the art that the present invention may be modified in various forms without departing from the spirit and scope of the appended claims. Moreover, the use of the terms first, second, etc., do not denote any order or importance, but rather the terms first, second, etc., are used to distinguish one element from another. Furthermore, the use of the terms a, an, etc., do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced item. 

1. A method for manufacturing a CoPtP alloy thin film comprising the steps of: preparing an electroplating solution comprising CoSO₄·7H₂O at 0.05˜0.2 M, H₂PtCl₆ at 0.005˜0.02 M, and NaH₂PO₂ at 0.01˜0.4 M; dipping a basic material into the electroplating solution; and forming the CoPtP alloy thin film on the basic material by electroplating at a low temperature.
 2. The method for manufacturing a CoPtP alloy thin film of claim 1, wherein the electroplating solution further comprises an ammonium salt at 0.1˜0.5 M in the step of preparing an electroplating solution.
 3. The method for manufacturing a CoPtP alloy thin film of claim 1, wherein the electroplating solution further comprises Na₄P₂O₇ at 0.3˜0.7 M in the step of preparing an electroplating solution.
 4. The method for manufacturing a CoPtP alloy thin film of claim 1, wherein the basic material is made of cobalt in the step of preparing an electroplating solution.
 5. The method for manufacturing a CoPtP alloy thin film of claim 1, wherein the electroplating is carried out at a temperature of 30˜55° C. in the step of forming the CoPtP alloy thin film.
 6. A CoPtP alloy thin film manufactured by using the method for manufacturing a CoPtP alloy thin film of claim
 1. 7. The CoPtP alloy thin film of claim 6, comprising: a plurality of CoPt grains; and boundary layers surrounding the CoPt grains, and wherein the boundary layers comprise Pt and P.
 8. The CoPtP alloy thin film of claim 7, wherein each of the CoPt grains are separated from each other by the boundary layers.
 9. The CoPtP alloy thin film of claim 6, with a coercivity of 4000 Oe˜7000 Oe.
 10. The CoPtP alloy thin film of claim 6, with a squareness of not less than 0.7. 